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The Mineral andalusite

Large Single Andalusite Crystal

Andalusite is known for its interesting variety Chiastolite, which contains a distinctive pattern within the cross-section of a crystal. The pattern is usually cross-shaped, and is caused by dark inclusions of carbon or clay. It is usually only visible when a crystal is cut. Andalusite is often replaced by other minerals, especially micas, Pyrophyllite and Kyanite, which can form a complete or partial pseudomorph after the Andalusite.

Andalusite is named after the Spanish province of Andalusia. The origin of the name is misleading, since the type locality where this mineral was first described is not Andalusia, but rather El Cardoso de la Sierra, which is well north of Andalusia in Guadalajara Province.

For additional information, see the gemstone section on Andalusite.

Chemical Formula

Al₂SiO₅

Color

Brown, greenish-brown, orange-brown, yellowish-brown, reddish-brown, light pink, yellow, gray, and white. A rare transparent form can be multicolored brown to yellow, and is strongly pleochroic.

Crystal System

Orthorhombic

Properties

Streak
Colorless

Hardness
7 - 7.5

Transparency
Transparent to opaque

Specific Gravity
3.1 - 3.2

Luster
Vitreous, dull

Cleavage
2,1

Fracture
Uneven, splintery

Tenacity
Brittle

Crystal Habits

In prismatic and blocky crystals and crystal grouping, often with a squared cross-section. The crystal shape is usually rectangular, and sometimes with beveled edges. Habits are most often massive, grainy, columnar, radiating, as embedded crystal outlines in matrix, and in rounded waterworn pebbles.

3D Crystal Atlas

Additional Information

Composition
Aluminum silicate

In Group
Silicates; Nesosilicates

Striking Features
Hardness, crystal habits, and patterns in the Chiastolite variety.

Environment
Andalusite is found in several environments, especially in metamorphosed schists, gneisses, and hornfels. Also in hydrothermal replacement deposits, granite pegmatites, and in alluvial deposits.

Rock Type
Igneous, Sedimentary, Metamorphic

Varieties

Chiastolite

- Andalusite with a distinctive cross pattern (or occasionally a checkerboard pattern) that forms in the interior cross-section of a crystal. Chiastolites are often cut into slices and polished. Although the species name has been discredited, it is still widely used and accepted by almost all collectors.

Viridine

- Bright green to olive-green variety of Andalusite, with its color supposedly caused by manganese impurities.

Uses

The rare transparent variety of Andalusite is used as a minor gemstone. Chiastolite cross-sections are often cut and polished to bring out the distinctive cross-shaped pattern, and Chiastolite crystals may also be cut along their cross-section and used in jewelry as a Christian symbol.

Noteworthy Localities

Well-formed Chiastolite crystals are abundant in the Boal area, Asturias, Spain; as well as in Carreço, Viana do Castelo, Portugal. Large, blocky crystals that have been pseudomophed by Pinite come from Lüsens, North Tyrol, Austria; and pink and peach colored crystals from Chiavenna and the Bregaglia Valley, Sondrio Province, Lombardy, Italy. An interesting radiating Andalusite within matrix comes from Dolní Bory, Moravia, Czech Republic.

Large Chiastolites have come from China at the Sangping Mine, Nanyang Henan Province; and from Australia at Mount Howden, Olary Province, South Australia. Brazil has several localities, with the gemmy multicolored variety coming from Santa Teresa Espirito Santo. Cinnamon-brown rectangular crystals of very good form have come from Itinga, in the Jequitinhonha valley, Minas Gerais.

In the U.S., some of the best Chiastolite has come from the Lancaster area, Worcester Co., Massachusetts. Very large Andalusite crystals were found in Alta Vista, Campbell Co., Virginia; and in the Hill City area, Pennington Co., South Dakota. In California, light pink and white Andalusite crystals have come from the Champion Mine, White Mountain, Laws, Inyo Co., California; and Chiastolite from Chowchilla, Madera Co.